CHAPTER 6

THE SECOND LAW OF

THERMODYNAMICS

Lecture slides by

Fawzi Elfghi

Thermodynamics: An Engineering Approach 8th Edition in SI Units

Yunus A. Çengel, Michael A. Boles

McGraw-Hill, 2015

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Objectives

• Introduce the second law of thermodynamics.

• Identify valid processes as those that satisfy both the first and second

laws of thermodynamics.

• Discuss thermal energy reservoirs, reversible and irreversible

processes, heat engines, refrigerators, and heat pumps.

• Describe the Kelvin–Planck and Clausius statements of the second law

of thermodynamics.

• Discuss the concepts of perpetual-motion machines.

• Apply the second law of thermodynamics to cycles and cyclic devices.

• Apply the second law to develop the absolute thermodynamic

temperature scale.

• Describe the Carnot cycle.

• Examine the Carnot principles, idealized Carnot heat engines,

refrigerators, and heat pumps.

• Determine the expressions for the thermal efficiencies and coefficients

of performance for reversible heat engines, heat pumps, and

refrigerators.

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PERPETUAL-

MOTION

MACHINES

Perpetual-motion machine: Any device that violates the first or the second law.

A device that violates the first law (by creating energy) is called a PMM1.

A device that violates the second law is called a PMM2.

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Despite numerous attempts, no perpetual-motion machine

is known to have worked.

If something sounds too good to be true, it probably is.

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REVERSIBLE AND IRREVERSIBLE PROCESSES

Reversible processes deliver the most

and consume the least work.

Reversible process: A process that can be reversed without leaving any trace

on the surroundings.

Irreversible process: A process that is not reversible.

• All the processes occurring in nature are irreversible.

• Why are we interested in reversible processes?

• (1) they are easy to analyze and (2) they serve as

idealized models (theoretical limits) to which actual

processes can be compared.

• Some processes are more irreversible than others.

• We try to approximate reversible processes. Why?

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Irreversibilities

Friction

renders a

process

irreversible.

Irreversible

compression

and

expansion

processes.

(a) Heat

transfer through

a temperature

difference is

irreversible, and

(b) the reverse

process is

impossible.

• The factors that cause a process to be

irreversible are called irreversibilities.

• They include friction, unrestrained expansion,

mixing of two fluids, heat transfer across a finite

temperature difference, electric resistance,

inelastic deformation of solids, and chemical

reactions.

• The presence of any of these effects renders a

process irreversible.

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Internally and Externally Reversible Processes

• Internally reversible process: If no irreversibilities occur within the boundaries of

the system during the process.

• Externally reversible: If no irreversibilities occur outside the system boundaries.

• Totally reversible process: It involves no irreversibilities within the system or its

surroundings.

• A totally reversible process involves no heat transfer through a finite temperature

difference, no nonquasi-equilibrium changes, and no friction or other dissipative

effects.

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THE CARNOT CYCLE

Reversible Isothermal Expansion (process 1-2, TH = constant)

Reversible Adiabatic Expansion (process 2-3, temperature drops from TH to TL)

Reversible Isothermal Compression (process 3-4, TL = constant)

Reversible Adiabatic Compression (process 4-1, temperature rises from TL to TH)

Execution of the Carnot cycle in a closed system.

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The Reversed Carnot Cycle

The Carnot heat-engine cycle is a totally reversible cycle.

Therefore, all the processes that comprise it can be reversed,

in which case it becomes the Carnot refrigeration cycle.

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THE CARNOT PRINCIPLES

1. The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs.

2. The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.

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THE THERMODYNAMIC

TEMPERATURE SCALE

A temperature scale that is

independent of the properties of

the substances that are used to

measure temperature is called a

thermodynamic temperature

scale.

Such a temperature scale offers

great conveniences in

thermodynamic calculations.

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This temperature scale is

called the Kelvin scale,

and the temperatures on

this scale are called

absolute temperatures.

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THE CARNOT HEAT ENGINE

Any heat

engine

Carnot heat

engine

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Analysis of a Carnot Heat Engine

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The Quality of Energy

How do you increase the thermal

efficiency of a Carnot heat engine?

How about for actual heat engines?

Can we use C unit

for temperature

here?

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THE CARNOT REFRIGERATOR AND HEAT PUMP

How do you increase the

COP of a Carnot

refrigerator or heat pump?

How about for actual ones?

Carnot refrigerator

or heat pump

Any refrigerator

or heat pump

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The COP of a reversible refrigerator or heat pump is the

maximum theoretical value for the specified temperature

limits.

Actual refrigerators or heat pumps may approach these

values as their designs are improved, but they can never

reach them.

The COPs of both the refrigerators and the heat pumps

decrease as TL decreases.

That is, it requires more work to absorb heat from lower-

temperature media.

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Heating a House by a

Carnot Heat Pump

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Summary • Introduction to the second law

• Thermal energy reservoirs

• Heat engines

Thermal efficiency

The 2nd law: Kelvin-Planck statement

• Refrigerators and heat pumps

Coefficient of performance (COP)

The 2nd law: Clausius statement

• Perpetual motion machines

• Reversible and irreversible processes

Irreversibilities, Internally and externally reversible processes

• The Carnot cycle

The reversed Carnot cycle

• The Carnot principles

• The thermodynamic temperature scale

• The Carnot heat engine

The quality of energy

• The Carnot refrigerator and heat pump